1. Field of the Invention
The invention relates to changeover valves for controlling fluid flow in a duct. In particular, the invention provides a changeover valve for installation in the intake duct of a variable cycle engine for a supersonic transport propulsion engine.
2. Description of Related Developments
An environmentally acceptable, and therefore commercially acceptable, propulsion unit for a second generation of supersonic transport aircraft must meet stringent noise regulations at take-off. To meet these requirements the engine configuration preferred currently is a variable cycle engine. Such an engine can meet the low noise requirements and, when demanded, product the high specific thrust needed for transonic acceleration. SST engine studies have concluded that a tandem fan type of engine is best suited to this role.
In a first, or parallel mode, the entire flow from the front fan is directed overboard and a secondary intake supplies air to a rear fan feeding directly to the engine core. This configuration is used during take-off, landing and subsonic flight to reduce noise levels. In effect, the engine in this configuration possesses a high bypass ratio, and low specific thrust.
For transonic and supersonic flight the engine is switched over to its second, or series, mode in which the changeover valve no longer dumps front fan airflow overboard. Instead it directs it to the rear fan thereby supercharging the core engine. Effectively this produces a low bypass ratio, high specific thrust engine possessing high cruise efficiency. This configuration is used where noise consideration are of a lesser concern.
A tandem fan engine of the type referred to is characterised by having an additional low pressure (LP) fan stage positioned ahead of a core engine with a changeover valve located in the intake duct in front of the engine but downstream of the additional fan. The changeover valve has to be capable of switching two engine airflows simultaneously, to allow the engine to changeover from operating in either of the two modes to operating in the other.
A valve of this type is described in our published European Patent Application No. EP 0412736A1. The changeover valve of that application essentially comprises a butterfly valve adapted to operate in an annular duct.
The secondary inlet and outlet arrangements are critical to efficient operation of this type of device. The simple arrangements disclosed in the earlier publication produce considerable turbulence. The secondary inlet is especially important in this respect since, in the parallel mode of operation, it is this airflow which enters the core engine. The present invention seeks to provide a solution to this problem.
The present invention has for a major objective to provide a secondary inlet arrangement which can supply flow with maximum efficiency to the core engine.
According to one aspect of the present invention there is provided a changeover valve for controlling flow in a duct comprising:
valve means for stemming flow along the duct;
secondary outlet means upstream of the valve means and operative in synchronism therewith to exhaust flow entering one end of the duct when said valve means blocks flow along the length of the duct;
secondary inlet means downstream of the valve means and operative in synchronism therewith a admit a secondary flow into the duct when said valve means blocks flow along the length of the duct;
said inlet means being formed by at least one aperture in a wall defining the duct and having closure means adapted for closing said at least one aperture, the closure means being hinged at an upstream edge to open into the duct.
Preferably, the secondary inlet arrangement includes inwardly hinged non-scoop doors located on opposite sides of the flow duct.
Preferably, the changeover valve further includes provision for a tertiary flow into the region behind the secondary inlet door or doors.